Would you try to....

[violet lightning]

It sounds like your friend might be open to something. He is "straight" yet is still crossdressing and from what you say, is putting some effort (sucessfully) into looking feminine, if only at Halloween.
Maybe you could just say something in a joking way like . "wow, you actually looked very convincing that night,...I was starting to get a little attracted to you!", and then gauge his reaction. If he looks intrigued, happy, etc, great, keep going. If not, just laugh it off as a joke and quickly change the subject.